Equalizing transceiver with reduced parasitic capacitance

ABSTRACT

A signaling circuit having reduced parasitic capacitance. The signaling circuit includes a plurality of driver circuits each having an output coupled to a first output node, and a plurality of select circuits each having an output coupled to a control input of a corresponding one of the driver circuits. Each of the select circuits includes a control input to receive a respective select signal and a plurality of data inputs to receive a plurality of data signals. Each of the select circuits is adapted to select, according to the respective select signal, one of the plurality of data signals to be output to the control input of the corresponding one of the driver circuits.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/261,875 filed Oct. 1, 2002 now U.S. Pat. No. 6,982,587, which claimspriority from U.S. Provisional Application No. 60/395,283 filed Jul. 12,2002.

FIELD OF THE INVENTION

The present invention relates generally to high speed signaling withinand between integrated circuit devices, and more particularly toreducing parasitic capacitance in an equalizing transceiver.

BACKGROUND

Equalizing driver circuits are often used in high-speed signalingsystems to mitigate the effects of inter-symbol interference (ISI) andinductive coupling between neighboring signal paths (i.e., crosstalk).FIG. 1 illustrates ISI in a prior-art signaling system in which data istransmitted as a series of distinct signal levels. At time T1, a logic 0signal is transmitted on a signal line by pulling the line up to levelV_(H). Subsequently, at time T2, a logic 1 is transmitted by pulling theline down to level V_(L). Finally, at time T3, a logic 0 is transmittedagain by pulling the signal line up to V_(H). Because of the signaldriving circuit has finite drive strength (i.e., finite ability to sinkand source current), the voltage level of the signal line does notchange instantaneously at time T2 or time T3, but rather exhibits afinite slew rate. Consequently, the ideal times for sampling (i.e., in areceiving circuit) the signals output at times T1, T2 and T3 occur atsample times S1, S3 and S3, respectively; after the signal hastransitioned to a relative minimum or maximum level and before thesignal begins transitioning to a next level. Referring to sample time S2in particular, note that the level of the signal is affected not only bythe logic 1 output at time T2, but also by the logic 0 output at time T1which, due to the finite slew rate of the transmitter, limits theability of the signal level to reach and settle at V_(L). The signal atsample time S2 is also affected by the logic 0 transmitted at time T3which limits the ability of the signal level to settle and hold atV_(L). Thus, values transmitted before and after the signal transmittedat time T2 interfere with the level of the T2 signal at the receiver dueto ISI.

FIG. 2 illustrates a prior-art output driver 100 in which ISI is reducedby dynamically increasing and decreasing the signal drive strength ofthe output driver 100 according to the relationship between past,present and future transmit data (TDATA). For example, if a logic 1 isto be transmitted (present data=1), but a logic zero was transmittedpreviously, the drive strength of the output driver 100 is temporarilyincreased to achieve faster slew from the logic 0 to logic 1 signallevels, thereby reducing the ISI caused by the previous transmission.Similarly, if a logic 1 is to be transmitted followed by a logic 0, thedrive strength of the output driver is temporarily increased to reducethe ISI caused by the subsequent transmission. Such dynamic adjustmentsto the drive strength of the output driver 100 are referred to asequalization operations, and the output driver is said to be anequalizing output driver.

The output driver 100 includes three sub-driver circuits formed byrespective current-sinking drive transistors (109, 111, 113) andcorresponding bias current sources (110, 112, 114). The sub-drivercircuits drive future, present and past data values, /A, B and /C,respectively (the ‘/’ symbol indicating complement), onto a signal path102 that is pulled up to a supply voltage through resistor, R.Flip-flops 105 and 107 are coupled in series to form a shift registerfor producing the present and past data values, B and /C, by shifting anincoming data signal, TDATA (i.e., /A), in response to a transmit clocksignal, TCLK. Thus, during a given cycle of the transmit clock signal,/A represents a data value to be transmitted in a subsequent cycle, Brepresents a data value to be transmitted in the present clock cycle,and /C represents a data value transmitted during the previous clockcycle. The bias currents produced by current sources 110, 112 and 114are 0.1I, 0.8I and 0.1I, respectively, so that the present data value,when high, draws current 0.8I (i.e., by switching on transistor 111) topull the output line 102 low, and the future and past values, when low,each draw current 0.1I (i.e., by switching on transistors 109 and 113,respectively) to pull the output line low 102 by incremental amounts.

FIG. 3 illustrates the effect of the future, present and past datavalues on the total current drawn by the prior-art output driver 100 ofFIG. 2. At time T1, the future, present and past data values (i.e.,A_(T1), B_(T1) and C_(T1)) are all zero so that, referring to FIG. 2,transistors 109 and 113 are switched on (i.e., due to the inversions ofvalues A and C), and transistor 111 is switched off. Accordingly, theoutput driver sinks a current of 0.2I to represent a steady-state logic0 condition and the voltage level of output line is pulled down slightlyto a nominal, V_(H) level. At time T2, the values of A, B, and C areshifted such that C_(T2)=B_(T1)=0, B_(T2)=A_(T1)=0, and A_(T2)=1. Inthis state, the current drawn by the output driver is reduced from 0.2Ito 0.1I to counteract the ISI that would otherwise result fromsubsequent transmission of a logic 1 value (i.e., at time T3).

At time T3, the values of A, B, and C are shifted again such thatB_(T3)=A_(T2)=1, C_(T3)=B_(T2)=0, and A_(T3)=1. Because B is high and Cis low, the output driver sinks a current of 0.9I; 0.8I via transistor111 and 0.1I via transistor 113. This current level may be understood byviewing the 0.8I drawn by transistor 111 as being a nominal currentneeded to produce the present logic 1 value, plus a current 0.1I drawnby transistor 113 to counteract the ISI from the logic 0 transmittedduring the preceding transmission interval.

At time, T4, the present, past and future values are all high (i.e.,A_(T4)=B_(T4)=C_(T4)=1), so that a current of 0.8I is drawn to representthe steady-state logic 1 condition. Finally, at time T5, the present andpast values remain at logic 1 (i.e., B=C=1), but the future value, A,becomes a logic 0. Consequently, the current drawn by the output driverincreases from 0.8I to 0.9I to counteract the ISI from the subsequentlogic 0 transmission.

Referring again to FIG. 2, signal equalization is achieved by the outputdriver 100 by driving the output signal line with two additionalsub-driver circuits (i.e., sub-driver circuits for past and futuredata). Because each sub-driver exhibits a parasitic capacitance, C_(i),the net affect of coupling additional sub-driver circuits to the outputsignal line is to increase the total parasitic capacitance of the outputdriver 100 from C_(i) to 3C_(i). This presents a significant problem inhigh-speed signaling systems, where the parasitic capacitance of theoutput driver tends to be a dominant, bandwidth-limiting capacitance ofthe signaling system. Additionally, transmission paths in high-speedsignaling systems are often terminated by termination elements havingimpedances selected to match the impedance of the transmission paths(i.e., as shown in FIG. 2, R is chosen to match Z₀), thereby reducingundesired signal reflections. The increased parasitic capacitance of theequalizing output driver produces a mismatch between the effectivetermination impedance and the transmission path impedance, therebyincreasing the level of signal reflections on the transmission path.Thus, it would be desirable to provide an equalizing output driverhaving reduced parasitic capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 illustrates inter-symbol interference in a prior-art signalingsystem;

FIG. 2 illustrates a prior-art output driver;

FIG. 3 illustrates the effect of the future, present and past datavalues on the total current drawn by the prior-art output driver of FIG.2;

FIG. 4 illustrates an equalizing output driver according to anembodiment of the invention;

FIG. 5 illustrates an exemplary embodiment of the data sub-driver ofFIG. 4;

FIG. 6 depicts a metal oxide semiconductor transistor;

FIG. 7 illustrates an embodiment of a push-pull sub-driver circuit;

FIG. 8 illustrates an embodiment of a differential pull-down sub-drivercircuit;

FIG. 9 illustrates an embodiment of an equalizing output driver forgenerating output signals having more than two possible states;

FIG. 10 illustrates an exemplary coding of input bit pair to acorresponding control signal within the equalizing output driver of FIG.9;

FIG. 11 illustrates the correspondence between bit-pair states andsignal levels in a multilevel signaling system;

FIG. 12 illustrates an equalizing output driver according to analternative embodiment of the invention;

FIG. 13 is a block diagram of an equalizing output driver having reduceddisparity between drive transistor sizes;

FIG. 14 illustrates an exemplary coding operation performed by thethermometer coding circuit of FIG. 13;

FIG. 15 illustrates an output sub-driver that may be driven by the codedcontrol value of FIG. 14;

FIG. 16 illustrates an equalizing output driver according to anotherembodiment of the invention;

FIG. 17 illustrates the operation of the equalizing output driver ofFIG. 16 in response to exemplary weighting values;

FIG. 18 illustrates an embodiment of an allocation logic circuit thatmay be used to implement the allocation logic of FIG. 16;

FIG. 19 illustrates a decoding operation performed within the allocationlogic circuit of FIG. 18;

FIG. 20 illustrates a shift operation performed within the allocationlogic circuit of FIG. 18;

FIG. 21 illustrates a logic operation within a select logic circuit ofFIG. 18;

FIG. 22 illustrates an exemplary embodiment of a select logic circuit;

FIG. 23 illustrates an equalizing receiver according to an embodiment ofthe present invention; and

FIG. 24 illustrates a signaling system in which an equalizing driverand/or equalizing receiver according to embodiments of the presentinvention may be used.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, specificnomenclature is set forth to provide a thorough understanding of thepresent invention. However, it will be apparent to one skilled in theart that these specific details may not be required to practice thepresent invention. In some instances, the interconnection betweencircuit elements or circuit blocks may be shown as multi-conductor orsingle conductor signal lines. Each of the multi-conductor signal linesmay alternatively be single signal conductor lines, and each of thesingle conductor signal lines may alternatively be multi-conductorsignal lines. A signal is said to be “asserted” when the signal isdriven to a low or high logic state (or charged to a high logic state ordischarged to a low logic state) to indicate a particular condition.Conversely, a signal is said to be “deasserted” to indicate that thesignal is driven (or charged or discharged) to a state other than theasserted state (including a high or low logic state, or the floatingstate that may occur when the signal driving circuit is transitioned toa high impedance condition, such as an open drain or open collectorcondition). A signal driving circuit is said to “output” a signal to asignal receiving circuit when the signal driving circuit asserts (ordeasserts, if explicitly stated or indicated by context) the signal on asignal line coupled between the signal driving and signal receivingcircuits. A signal line is said to be “activated” when a signal isasserted on the signal line, and “deactivated” when the signal isdeasserted. Additionally, the prefix symbol “/” attached to signal namesindicates that the signal is an active low signal (i.e., the assertedstate is a logic low state). A line over a signal name (e.g., ‘ <signalname>’) is also used to indicate an active low signal. Active lowsignals may be changed to active high signals and vice-versa as isgenerally known in the art.

Equalizing Output Driver Having Reduced Parasitic Capacitance

Equalizing output driver circuits having reduced parasitic capacitanceare disclosed herein in various embodiments. In one embodiment,equalizing sub-driver circuits within an output driver are scaledrelative to a primary signal driver to reduce the parasitic capacitanceof the equalizing sub-drivers, and thereby reduce the total parasiticcapacitance of the output driver. In another embodiment, weightedequalization values are summed with a weighted primary signal valueduring each data transmission cycle to produce a drive strength controlvalue. The drive strength control value is applied to an output drivercircuit to achieve a signal drive strength that reflects theequalization values and primary signal value. Because the weightedequalization and primary signal values are summed in the digital domain,separate equalizing sub-driver circuits are unnecessary, and may beomitted to reduce the overall parasitic capacitance of the outputdriver. In another embodiment, an equalizing output driver includessub-driver circuits that are allocated among primary and equalizingdriver pools according to a set of configuration values. Because thesub-driver circuits are, in effect, shared among the primary andequalizing driver groups, the total number of sub-driver circuits isreduced relative to the number of sub-driver circuits otherwise neededto achieve the same range of primary and equalizing signalcontributions. The reduced number of sub-driver circuits results in acorrespondingly reduced input capacitance.

Equalizing Output Driver Having Scaled Equalizing Sub-Drivers

FIG. 4 illustrates an equalizing output driver 200 according to anembodiment of the invention. The equalizing driver 200 includes aplurality of sub-driver circuits 203, 205 and 207 (i.e., sub-drivers),two of which (203 and 207) are used to contribute equalization levels toa signal output via pad 201, and are therefore referred to herein asequalizing sub-drivers. The remaining sub-driver circuit, 205, is usedto drive the output signal according to the data value to be transmitted(i.e., primary data) and is referred to herein as a data sub-driver. Inthe embodiment of FIG. 4, the equalizing sub-drivers 203 and 207 mayfurther be distinguished according to their data sources. Equalizingsub-driver 207, for example, is responsive to a data value, C, that wastransmitted by the data sub-driver 205 in a previous transmissioninterval (i.e., past data, referred to herein as post-tap data) and istherefore referred to herein as a post-tap sub-driver, or post-tap.Equalizing sub-driver 203, by contrast, is responsive to a data value,A, to be transmitted by the data driver 205 in a subsequent transmissioninterval (i.e., future data, referred to herein as pre-tap data) and istherefore referred to herein as a pre-tap sub-driver, or pre-tap. In theembodiment of FIG. 4, a single pre-tap sub-driver and a single post-tapsub-driver are shown. In alternative embodiments, any number of pre-tapand post-tap sub-drivers may be provided. In one embodiment, forexample, three post-tap sub-drivers (each responding to a successivelydelayed post-tap datum) and one pre-tap sub-driver are provided. Inother embodiments, one or more post-tap sub-drivers may be provided andpre-tap sub-drivers may be omitted entirely. Conversely, one or morepre-tap sub-drivers may be provided and post-tap sub-drivers omittedentirely. In much of the remaining description, embodiments having asingle pre-tap sub-driver and a single post-tap sub-driver aredescribed. In all such embodiments, more or fewer pre-tap and post-tapsub-drivers may be provided. Also, the data sub-driver 205 may beomitted, particularly when the equalizing output driver 200 is usedwithin an equalizing receiver, as discussed below.

Still referring to FIG. 4, pre-tap data value, A, is provided to thepre-tap sub-driver 203, in complement form (i.e., /A) via a signal line204. A delay element 215 (e.g., a flip-flop, latch, delay circuit, etc.)provides a controlled delay (i.e., 1/Z) between the pre-tap and primarydata values, A and B. Similarly, a delay element 217 provides acontrolled delay between the primary and post-tap data values, B and C.Additional delay elements may be coupled in the data path (i.e., thesignal path carrying TDATA) prior to delay element 215 to generateadditional pre-tap data values, and additional delay elements may becoupled in the data path after the delay element 217 to generateadditional post-tap data values. Pre-tap and post-tap data values mayalso be provided as either inverted or non-inverted data. In theembodiment of FIG. 4, the delay elements 215 and 217 producecomplemented outputs such that, if the pre-tap data value A is drivenonto the data line 204 in complemented form, primary data value B isprovided to data sub-driver 205 via signal line 206 in uncomplementedform, and post-tap data C is provided to equalizing sub-driver 207 viasignal line 208 in complemented form. By this arrangement, theequalizing sub-drivers 203 and 207 contribute to the combined outputsignal in a manner that counteracts differences between the primary datavalue B and the pre- and post-tap data values, respectively.

In the embodiment of FIG. 4, each of the data and equalizing sub-drivers203, 205 and 207 is implemented by a single-ended, pull-down sub-drivercircuit. The data sub-driver 205, for example, includes a current source212 controlled by a bias signal, S_(B), and a switching transistor 211that switches the data sub-driver 205 between on and off states inresponse to high and low logic levels, respectively, of the primary datavalue, B. Thus, when B is high, transistor 211 is switched on, enablingcurrent I to be drawn from an output line (i.e., via pad 201), therebypulling down the level of output line 202 (which may be pulled up, forexample, by connection via a termination element or circuit).Conversely, when B is low, transistor 211 is switched off, and nocurrent is drawn by the data sub-driver. The equalizing sub-drivers 203and 207 are similarly implemented by a current sources (210, 214) and aswitching transistor (209, 213). The current source 210 is controlled bybias signal, S_(A), and the current source 214 is controlled by currentsource, S_(B).

FIG. 5 illustrates an exemplary embodiment of the data sub-driver 205 ofFIG. 4. The data sub-driver includes the switching transistor 211 andcurrent source 212 as described above. The current source 212 includesmultiple transistors 227 ₀-227 ₅ coupled in parallel between a referencevoltage (ground in this example) and a source terminal of the switchingtransistor 211. Gate terminals of the transistors 227 ₀-227 ₅ arecoupled to receive respective component signals, S_(B)[0]-S_(B)[5], ofthe bias signal, S_(B). Each of the transistors 227 has a binaryweighted gain such that a current of I_(REF)×2^(i) (where i representsthe i^(th) transistor in the positions 0, 1, 2, 3, 4, 5) flows throughtransistor 227 _(i) when the corresponding bias signal component and thedata value, B, are both high. That is, assuming that B is high and thatall the bias signal components SB[5]-SB[0] are high, then I_(REF) flowsthrough transistor 227 ₀, I_(REF)×2 flows through transistor 227 ₁,I_(REF)×4 flows through transistor 227 ₂, I_(REF)×8 flows throughtransistor 227 ₃, I_(REF)×16 flows through transistor 227 ₄, andI_(REF)×32 flows through transistor 227 ₅. Accordingly, transistors 227₀-227 ₅ are designated ×1, ×2, ×4, ×8, ×16 and ×32 transistors,respectively. By this arrangement, the bias signal componentsSB[5]-SB[0] may be set to any of 2⁶ binary patterns to select biascurrents that range from 0 to I_(REF)×63 in increments of I_(REF). Inthe embodiment of FIG. 5, the switching transistor 211 is designed todeliver a current of I_(REF)×64; a current substantially equal to themaximum current that can be drawn by the current source 212. Inalternative embodiments, the current source 212 may have more or fewerbinary weighted transistors (i.e., to enable selection of more or fewerbias currents) and the switching transistor 211 may be scaled to delivermore or less current, accordingly.

In one embodiment of the FIG. 4 output driver, metal oxide semiconductor(MOS) transistors are used to implement the switching transistor andcurrent sources within the sub-drivers 209, 211 and 213, and therelative gains (i.e., transconductance values) of the variousimplementing transistors (and therefore drive strengths of thesub-drivers) are established by adjusting the width-length ratio (i.e.,W/L) of individual transistors. Referring to FIG. 5, for example, thewidth-length ratio of the ×2 transistor 227 ₁ is twice the width-lengthratio of the ×1 transistor 227 ₀, the width-length ratio of the ×4transistor 227 ₂ is twice the width-length ratio of the ×2 transistor227 ₁, and so forth. Referring to FIG. 6, which depicts a MOS transistor249 having a source terminal 252 (S), drain terminal 254 (D), gateterminal 256 (G), and body 250 (B), it can be seen that a primary sourceof parasitic capacitance, C_(i), occurs at the drain-to body junction(the body 250 forming a dielectric, for example, between the drainterminal 254 and a ground plane). Accordingly, the smaller the area ofthe drain terminal 254, the lower the parasitic capacitance of thetransistor 249. Thus, the width of the transistor 249 may be reduced toproduce a corresponding reduction in parasitic capacitance, C_(i). Thisrelationship between parasitic capacitance and transistor width isexploited in the embodiment of FIG. 4 to achieve an overall reduction inthe parasitic capacitance of the output driver 200. More specifically,the equalizing sub-drivers 203 and 207 are implemented in the samemanner as the data driver 205 (e.g., as shown in FIG. 5), except thatthe widths of the switching transistors 209 and 213 within theequalizing sub-drivers 209 and 213 are reduced by scaling factors K_(A)and K_(C), respectively, to achieve corresponding reductions in theequalizing sub-driver contributions to the overall parasitic capacitanceof the output driver 200. That is, instead of using identicalsub-driving circuits for the equalizing and data sub-drivers, (whichwould yield a combined parasitic capacitance of three times thecapacitance, Ci, of the data sub-driver), reduced-width transistors areused to implement the switching transistors 209 and 213 within theequalizing sub-drivers 203 and 207, thereby yielding a combinedparasitic capacitance that is less than 3C_(i). More specifically,because the parasitic capacitance of the switching transistors issubstantially proportional to the width of the transistors, the combinedC_(i) of the driver circuit 200 is substantially equal toC_(i)+K_(A)C_(i)+K_(C)C_(i), where the scaling factors, K_(A) and K_(C),are each less than one.

Still referring to FIG. 4, the reduced width of the switchingtransistors 209 and 213 produces a corresponding transistor gainreduction and therefore reduced drive strength in the equalizingsub-drivers 203 and 205. In one embodiment, the scaling factors K_(A)and K_(C) are selected according to the maximum anticipated current drawwithin the equalizing sub-drivers 203 and 205. For example, if thepre-tap sub-driver 203 is anticipated to draw a maximum current equal to25% of the data sub-driver current, the pre-tap scaling factor, K_(A),may be selected to be 0.25. The post-tap scaling factor, K_(C), may bedetermined in a similar manner.

Referring again to FIG. 5, it should be noted that, in the case of ascaled post-tap or pre-tap sub-driver (i.e., having reduced width andtherefore reduced-gain switching transistor), the maximum bias currentdrawn by the sub-driver current source may be correspondingly scaled.For example, if the switching transistors 209 and 213 of the pre-tap andpost-tap sub-drivers are each scaled to have a gain scaled to 0.25 timesthe data driver gain, the corresponding current sources 210 and 214 mayeach be implemented by omitting transistors 227 ₅ and 227 ₄, therebyproviding for a maximum bias current substantially equal to 0.25 timesthe maximum bias current of the data sub-driver 205. Scaled biascurrents within the equalizing sub-drivers 203 and 207 of FIG. 4 areindicated by the bias current designations K_(A)I and K_(C)I.

Although the equalizing output driver of FIG. 4 has been described interms of single-ended, pull-down sub-driver circuits (203, 205, 207),virtually any type of sub-driver circuit may be used in alternativeembodiments. For example, FIG. 7 illustrates an embodiment of apush-pull sub-driver circuit 273 that sources or sinks current (therebypulling an output signal (OUT) high or low) according to the level of aninput signal (IN) and which may be used in place of the pull-downsub-driver circuits 203, 205, 207 of FIG. 4. In such an embodiment, thedrain terminals of component transistors 275 and 277 may be scaledwithin the equalizing sub-driver circuits to achieve scaled parasiticcapacitance. Also, two such push-pull sub-drivers 273 alternativelycoupled to IN+ and IN− input signals may be used to implement adifferential push-pull sub-driver. FIG. 8 illustrates an embodiment of adifferential pull-down sub-driver 283 that may be used in place of thesingle-ended pull-down sub-drivers 203, 205 and 207 of FIG. 4. Thedifferential pull-down sub-driver 283 includes a pair of transistorshaving gate terminals coupled to receive differential input signals, IN+and IN−, and which therefore alternately pull-down output signal lines(OUT− and OUT+) coupled to drain terminals of switching transistors 285and 287 via resistive pull-up elements, R. Note that the pull-upelements may be implemented by passive or active components, and may be,for example, termination resistances coupled to the output signal lines.As with the switching transistors within the sub-driver circuits of FIG.4, the width-length ratios of the differentially coupled switchingtransistors 285 and 287 may be scaled within equalizing sub-drivers toreduce the total parasitic capacitance of the equalizing output driver.Thus, while single-ended pull-down sub-drivers are described inreference to FIG. 4 and in embodiments described below, virtually anytype of sub-driver circuit, including combinations of different types ofsub-driver circuits, may alternatively be used in such embodimentswithout departing from the spirit and scope of the present invention.

Referring to FIGS. 4-8, it should be noted that while embodimentsimplemented by MOS transistors have been described, other processtechnologies (e.g., bipolar, gallium-arsenide, etc.) may be used toimplement the sub-driver circuits of an equalizing driver. Moregenerally, though MOS circuits are described in reference to FIGS. 4-8and in embodiments described below, any process technology mayalternatively be used in such embodiments without departing from thespirit and scope of the present invention.

Equalizing, Multi-Level Output Driver

FIG. 9 illustrates an embodiment of an equalizing output driver forgenerating output signals having more than two possible states (referredto herein as multi-level signals). The equalizing output driver includesthree multi-level sub-driver circuits each of which receives two databits and generates, in response, an output signal having one of foursignal levels. In one embodiment, the output driver is coupled, via pad##, to a pulled-up signal line (not shown) such that each of fourdifferent output current levels pulls the signal line down to one offour different voltage levels. Referring to FIG. 11, for example, thefour possible states of data bits B[1:0] correspond to four differentdrive current levels (e.g., 00→0I, 01→I/3, 10→2I/3, and 11→I, thoughdifferent codings may be used) and therefore to four different voltagelevels: V_(HH), V_(MH), V_(ML) and V_(LL). A multilevel signal receivermay distinguish between the four different voltage levels by comparingan incoming multilevel signal against three threshold voltages set tothe respective midpoints of the three voltage steps between the V_(HH)and V_(LL).

Still referring to FIG. 9, each of the equalizing sub-drivers and thedata sub-driver includes a coding circuit 308 and a set of componentsub-drivers. Each of the component sub-drivers is implemented by aswitching transistor and adjustable current source in the same manner asdescribed above in reference to FIG. 4, except that the width-lengthratios of each of the switching transistors (and the maximum selectablecurrent through the current sources) is reduced by a factor of three.For example, the width-length ratio of each of the switching transistors329, 331 and 333 of data sub-driver 305 is ⅓ the width-length ratio ofthe switching transistor 211 within the data sub-driver 205 of FIG. 4,and the bias current drawn by each of the three current sources 330, 332and 334 within the data sub-driver 305 is ⅓ the bias current drawn bythe current source 212 of FIG. 4. By this arrangement, when all threeswitching transistors within the data sub-driver 305 are switched on,the data sub-driver 305 draws a current substantially equal to I (i.e.,I/3+I/3+I/3). The switching transistors (319, 321 and 323) and currentsources (320, 322, and 324) within the pre-tap sub-driver 303, and theswitching transistors (339, 341 and 343) and current sources (340, 342and 344) within the post-tap sub-driver 307 are similarly scaled by afactor of three relative to their counterparts in the pre- and post-tapsub-drivers 203 and 207 of FIG. 4.

The coding circuits 308 ₁, 308 ₂ and 308 ₃ each respond to a respectiveone of the input bit pairs, /A[1:0], B[1:0] and /C[1:0], by generating acorresponding 3-bit control signal, M_(A)[2:0], M_(B)[2:0] andM_(C)[2:0]. FIG. 10 illustrates an exemplary coding of input bit pairB[1:0] to a corresponding control signal, M_(B)[2:0]. Input bit pairs /Aand /C may be similarly coded to produce control signals M_(A) andM_(C). Also, other coding schemes may be used in alternativeembodiments. Referring to FIGS. 9 and 10, it can be seen that adifferent number of component sub-driving circuits is enabled within thedata sub-driver 305 for each different state of input bit pair B[1:0].Specifically, when B[1:0]=00, all three component sub-drivers areswitched off (i.e., MB[2:0]=000, switching off transistors 329, 331 and333), so that the data sub-driver 303 draws zero current (i.e., I=0).When B[1:0]=01, one of the three component sub-drivers is switched on todraw current, I/3; when B[1:0]=10, two of the three componentsub-drivers are switched on to draw combined current, 2I/3; and whenB[1:0]=11, all three of the component sub-drivers are switched on todraw a combined, full-scale current, I. In this way, the four differentcurrent levels (and voltage levels) described in reference to FIG. 11may be achieved. Also, the equalizing sub-drivers 303 and 307 similarlycontribute equalizing currents according to the pre- and post-tap databit pairs, A[1:0] and C[1:0]. Specifically, the pre-tap sub-driver 303contributes an equalizing current that ranges from 0 to K_(A)I in stepsof K_(A)I/3, and post-tap sub-driver 307 contributes an equalizingcurrent that ranges from 0 to K_(C)I in steps of K_(C)I/3.

In one embodiment, the switching transistors 329, 331 and 333 within thedata sub-driver 305 are scaled by transistor width reduction, so thatthe parasitic capacitance of each component sub-driving circuit withinthe data sub-driver 305 is substantially equal to one-third theparasitic capacitance of the data sub-driver 205 of FIG. 4 (and so thatthe total parasitic capacitance of the data sub-driver 305 issubstantially equal to the parasitic capacitance, C_(i), of the datasub-driver 205 of FIG. 4. The switching transistors and (and currentsources) within the equalizing sub-drivers 303 and 307 are similarlyscaled by a factor of three relative to their counterparts in theequalizing sub-drivers 203 and 207 of FIG. 4. Accordingly, the totalparasitic capacitance of the multi-level output driver 300 of FIG. 9 issubstantially equal to the total parasitic capacitance of output driver200 of FIG. 4. That is, the data sub-driver 305 exhibits a full-scaleparasitic capacitance of C_(i), while the equalizing sub-drivers 303 and307 each exhibit parasitic capacitances that are reduced relative toC_(i), by factors of K_(A) and K_(C), respectively.

Comparing the architectures of the equalizing drivers of FIGS. 4 and 9,it can be seen that the sub-driver interconnections between the outputpad 201 and the data source is essentially identical, except thatmultiple bits are provided to each of the multilevel sub-drivers 303,305 and 307. Thus, any binary-level output driver described herein mayreadily be adapted for use in a N-level signaling application by codingmultiple data bits (and/or pre-tap bits and/or post-tap bits) togenerate N−1 control signals, and by further subdividing eachsub-driving circuit into N−1 component sub-driving circuits, eachcomponent sub-driving circuit being scaled by a factor of 1/(N−1) andcontrolled by a respective one of the N−1 control signals.

Output Driver with Digital-Domain Equalization

FIG. 12 illustrates an alternative embodiment of an equalizing outputdriver 370 having reduced parasitic capacitance. The equalizing driver370 includes a summing circuit 371, pre-driver 373, and outputsub-driver 375. The summing circuit 371 includes multiplexers 387, 389and 391, and adder 381 to combine the output signal contributionsindicated by bias control values S_(A), S_(B) and S_(C), according tothe states of the corresponding pre-tap, primary and post-tap datavalues, A, B and C. The multiplexer 387 outputs the pre-tap bias controlvalue S_(A) to the adder 381 if pre-tap data value, A, is low (i.e.,if/A is high), and otherwise passes a zero value to the adding circuit.Similarly, multiplexer 389 outputs either the post-tap bias controlvalue, S_(C), or a zero value to the adder 381 according to whether thepost-tap data value, C, is low or high, respectively, and multiplexer391 outputs either the primary bias control value, S_(B), or a zerovalue to the adder 381 according to whether the primary data value, B,is high or low, respectively. The adder 381 sums the values output bythe multiplexers 387, 389 and 391 to generate an N-bit summed controlsignal 390, R[N−1:0], that represents the summed, weighted contributionsof the pre-tap, primary and post-tap values. The N constituent bits ofthe summed control signal 390 are amplified by N amplifiers, 393 ₀-393_(N−1), within the pre-driver 373, then applied to gate terminals ofrespective binary weighted drive transistors, 395 ₀-395 _(N−1), withinthe output sub-driver 375 to achieve an equalized output signal. Becausethe contributions of the pre-tap and post-tap data values are applied inthe digital domain (e.g., by logic within the summing circuit 371), asingle sub-driver 375 may be used (i.e., as opposed to providingseparate output drivers for pre-tap and post-tap equalization purposes).Thus, even though the output sub-driver 375 includes multipletransistors 395 coupled to the output pad 201, the maximum current drawnby the output sub-driver is nominally the same, for example, as themaximum current, I, drawn by the data sub-driver 205 of FIG. 4. That is,the width-length ratios of the N binary weighted drive transistors 395are such that the largest drive transistor 395 _(N−1) has one-half thewidth-length ratio of the switching transistor 211 of the FIG. 4 datasub-driver, the next largest drive transistor 395 _(N−2) has one fourththe width-length ratio of the switching transistor 211 and so forth suchthat the combined size of all the transistors 395 _(N)-1-395 ₀ issubstantially the same as the size of the switching transistor 211.Accordingly, the total parasitic capacitance of the equalizing driver370 is roughly equal to the parasitic capacitance, C_(i), of the FIG. 4data sub-driver 205 alone.

Still referring to FIG. 12, the larger drive transistors within theoutput sub-driver 376 tend to have a larger gate capacitance than thesmaller drive transistors and therefore require greater charge transferto the gate terminal in order to achieve the same operating point.Accordingly, in the embodiment of FIG. 12, the pre-drive amplifiers 393_(N−1)-393 ₀ within the pre-driver 373 are designed to have differentdrive strengths (i.e., gains) according to the width/length ratio of thedrive transistor to be controlled. For example, the amplifier 393 _(N−1)has a greater drive strength (i.e., signal gain) than amplifier 393_(N−2); amplifier 393 _(N−2) has a greater drive strength than amplifier393 _(N−3) and so forth. By implementing pre-drive amplifiers 393 withdifferent drive strengths in this manner, each of the signal drivingtransistors 395 within the output sub-driver 375 may be switched from anoff condition to a desired operating point (e.g., in saturation) insubstantially the same amount of time.

Returning briefly to FIG. 4, it should be noted that, depending on thesize difference between the switching transistor 211 within the datasub-driver and the switching transistors 209 and 213 within the pre-tapand post-tap sub-drivers, it may also be desirable to providedifferent-strength pre-drive amplifiers to drive the pre-tap, post-tapand data values to the gates of switching transistors 209, 211, 213,respectively.

Referring again to FIG. 12, numerous types of circuits may be used toimplement the adder 381 including, without limitation, combinatoriallogic, a dedicated state machine, a general purpose processor, a digitalsignal processor, etc. More generally, any circuitry capable ofselectively adding the bias control values S_(A), S_(B) and S_(C) (i.e.,according to the states of the corresponding data values) may be used toimplement the summing circuit 371. Also, the summing circuit 371 mayreadily be adapted to sum the pre-tap and post-tap contributions ofmultiple bits, for example, where there are more or fewer pre- and/orpost-tap values than shown in FIG. 12, or where multi-level outputsignals are to be generated.

Referring again to the output sub-driver 375, because each of the drivetransistors 395 ₀-1-395 _(N−1) have finite output resistance (i.e., thedrain voltage increases with drain-to-source current, even insaturation) and because the gains of each of the transistors aredifferent, it may be difficult to achieve precisely the same outputvoltage at the drain terminal of each drive transistor 395. Theresulting voltage differentials between the drain terminals of the drivetransistors 395 may result in undesirable distortion of the outputsignal. In one embodiment, this distortion is substantially reduced bythermometer coding the most significant bits of the summed controlsignal 390 and distributing the drive responsibility of the highest-gainsub-driver transistors (the primary distortion contributors) amongmultiple, smaller-gain drive transistors. FIG. 13 is a block diagram ofan equalizing output driver 400 according to such an embodiment. Asshown, the equalizing output driver 400 includes a summing circuit 371,thermometer coding circuit 403, pre-driver 405 and output sub-driver407. The summing circuit 371 operates as described in reference to FIG.12 to generate an N-bit summed control value 390, R[N−1:0]. Thethermometer coding circuit 403 decodes a selected number of the mostsignificant bits of the control signal 390 to generate a K-bit codedcontrol value, CS. The coded control value and least significant bits ofthe summed control value 390 are amplified by the pre-driver 405 (i.e.,according to size differences of drive transistors within the outputdriver) and output to constituent drive transistors within the outputsub-driver 407. It should also be noted that the implementation can bedone with or without any combination of the thermometer coding circuitryand the pre-driver circuitry.

FIG. 14 illustrates an exemplary coding operation performed by thethermometer coding circuit of FIG. 13. For purposes of example only, thesummed control value is assumed to be a six bit value, R[5:0], in whichthe most significant three bits, R[5:3], are coded to generate a sevenbit coded control value CS[9:3]. In the exemplary coding depicted, thecoded control value, CS[9:3], includes a number of logic high bitsaccording to the numeric value of summed control bits R[5:3]. That is,if bits R[5:3]=000, then none of the bits CS[9:3] are high; ifR[5:3]=001 (decimal 1), then one of the bits CS[9:3] is high; ifR[5:3]=010 (decimal 2), then two of the bits CS[9:3] is high, and soforth.

FIG. 15 contrasts an output sub-driver 420 that may be driven by the6-bit summed control value of FIG. 14 and an output sub-driver 422 thatmay be driven by a combination of the coded control value of FIG. 14 andthe least significant bits of the summed control value. As shown, thethree largest drive transistors, 421 ₅-421 ₃ (i.e., the ×32, ×15 and ×8transistors), within the output sub-driver 420 are replaced in theoutput sub-driver 422 by seven drive transistors, 423 ₆-423 ₀, eachhaving a ×8 drive strength. Because the number of high CS bits coupledto the seven ×8 drive transistors is equal to the value of the R[5:3]bits, a number of the ×8 transistors within the output sub-driver 422are turned on in accordance with the value of the R[5:3] bits. Thus, theoutput sub-driver 422 exhibits a drive strength equal to that of outputsub-driver 420 (i.e., for a given value of the R[5:3] bits) using drivetransistors no larger than ×8. Consequently, the output distortioncaused by differences in drive transistor sizes in is reduced relativeto the output sub-driver 420. The least significant three bits of thesummed control value, R[2:0], are used to drive the smaller transistors,421 ₂-421 ₀, within each of the output sub-drivers 420 and 422. Notethat specific numbers of control signal bits and drive transistors havebeen described in reference to FIGS. 14 and 15 for purpose of exampleonly. Different numbers of control signal bits and drive transistors maybe used in alternative embodiments. Also, more or fewer of the mostsignificant bits of the summed control value, R, may be coded inalternative embodiments. Further, coding schemes other than that shownin FIG. 14 may be used in alternative embodiments.

Equalizing Output Driver with Allocated Sub-Drivers

FIG. 16 illustrates an equalizing output driver 470 according to anotherembodiment of the invention. The equalizing output driver 470 includesan allocated driver circuit 471 and dedicated driver circuit 473, andreceives primary data value B, and pre- and post-tap data values A and Cas inputs. The equalizing output driver 470 additionally receivesmulti-bit weight values, W_(A), W_(B) and W_(C), as inputs. The weightvalues represent the relative output signal contributions of primary andequalizing data values during each transmission interval, and may beprovided by a configuration circuit (not shown) within an integratedcircuit containing the equalizing output driver 470 or, alternatively,by an off-chip source including, without limitation, another integratedcircuit device or printed circuit board strapping. The least significantbits (LSBs) of the weight values, W_(A), W_(B) and W_(C), are suppliedto the dedicated driver circuit which, in response, outputs anequalized, least-significant-bit (LSB) signal to pad 201 via signal line474. The most significant bits (MSBs) of the weight values are providedto the allocated driver circuit 471 which, in response, allocatessub-drivers 495 within the allocated driver circuit 471 amongdata-driving, and pre- and post-tap driver pools. That is, the allocateddriver circuit 471 enables a sub-driver 495 not needed for equalizationpurposes to be used as a data sub-driver (and vice-versa), therebylowering the overall number of sub-drivers 495 that would be necessaryto achieve the same range of data and equalizing drive strengths inabsence of such sub-driver allocation. The reduced number of sub-drivers495 coupled to the output pad 201 (i.e., via signal path 472) results ina corresponding reduction in parasitic capacitance of the equalizingoutput driver 470.

In the exemplary embodiment of FIG. 16, each of the weight values,W_(A), W_(B) and W_(C), are 7-bit values, the most significant threebits of which are provided to the allocated driver circuit 471 and theleast four significant bits of which are provided to the dedicateddriver circuit 473. The weight values may include more or fewer bits inalternative embodiments, and the distribution of the constituent bits ofthe weight values between the allocated and dedicated driver circuitsmay be different. The allocated driver circuit 471 includes allocationlogic 493 which responds to the most significant bits of the weightvalues by generating a multi-bit allocation control signal, AC. In theembodiment of FIG. 16, the allocation control signal is a fourteen-bitsignal (more or fewer bits may be used in alternative embodiments) inwhich respective groups of two bits are coupled to select inputs ofmultiplexers 497 ₀-497 ₆. That is, allocation control bit pair AC₀[1:0]is coupled to the select input of multiplexer 497 ₀, allocation controlbit pair AC₁[1:0] is coupled to the select input of multiplexer 497 ₁,and so forth to allocation control bit pair AC₆[1:0] which is coupled tothe select input of multiplexer 497 ₆. Each of the multiplexers 497includes four input ports (designated ‘00’, ‘01’, ‘10’ and ‘11’ in FIG.16) coupled respectively to receive a logic low signal, complementedpre-tap data value (/A), primary data value (B), and complementedpost-tap data value (/C). Each of the sub-drivers 495 ₀-495 ₆ includes aswitching transistor (498 ₀-498 ₆, respectively) having a gate terminalcoupled to the output of a respective one of the multiplexers 497 ₀-497₆, and a current source (499 ₀-499 ₆, respectively) biased to drawcurrent, I_(REF)×16. By this arrangement, each of the sub-drivers 495may selectively be controlled by either a pre-tap data value, /A,primary data value, B, or post-tap data value, /C. Each sub-drivercircuit 495 selected to be controlled by a pre-tap data value isreferred to as a pre-tap sub-driver and is said to be allocated to apre-tap pool (the pre-tap pool including one or more pre-tapsub-drivers). Similarly, each sub-driver 495 selected to be controlledby a post-tap data value is referred to as a post-tap sub-driver and issaid to be allocated to a post-tap pool, and each sub-driver 495selected to be controlled by a primary data value is referred to as adata sub-driver and is said to be allocated to a data driver pool. Thus,each of the sub-drivers 495 within the allocated driver circuit may beallocated to a pre-tap, post-tap or data driver pool, with theallocation in a given application being determined by the allocationsignal, AC, and therefore by the most significant bits of the weightvalues, W_(A), W_(B) and W_(C). In the embodiment of FIG. 16, anyunallocated sub-driver 495 (i.e., sub-driver not needed within thepre-tap, post-tap or data driver pools) is disabled by selection of theground reference input to port ‘00’ of the corresponding multiplexer497. The current source 499 within each unallocated sub-driver 495 mayalso be disabled.

The dedicated driver circuit 473 includes a dedicated data sub-driver477, dedicated pre-tap sub-driver 475 and dedicated post-tap sub-driver479, all implemented generally as described in reference to FIG. 4,except that the pre- and post-tap sub-drivers 475 and 479 are not scaled(i.e., the switching transistors and current sources of the data, pre-and post-tap sub-drivers have the same current sinking capability).Also, the least significant bits (LSBs) of weight values, W_(B), W_(A)and W_(C), constitute the bias control signals for the current sourceswithin the data sub-driver 477, pre-tap sub-driver 475 and post-tapsub-driver 479, respectively. In the embodiment of FIG. 16, theswitching transistors 485, 481 and 489 within the data sub-driver 477,and pre-tap sub-driver 475 and post-tap sub-driver 479, respectively,each have substantially the same width-length ratio as the switchingtransistors 498 within the sub-drivers 495 of the allocated drivercircuit (e.g., ×16 transistors). By this arrangement, all thetransistors coupled to pad 201 within the equalizing output driver 470have substantially the same size, thereby avoiding the distortion thatmay occur when differently sized transistors are used. In the embodimentof FIG. 16, the current sources 487, 483 and 491 within the datasub-driver 477, pre-tap sub-driver 475 and post-tap sub-driver 479 eachinclude four binary weighted transistors (as shown in expanded viewwithin pre-tap sub-driver 475) having drive strengths I_(REF)×1, ×2, ×4and ×8. Accordingly, a bias current ranging from 0 to I_(REF)×15 insteps of I_(REF) may be selected within the data sub-driver 477, andpre- and post-tap sub-drivers 475 and 479 according to the LSBs of theweight values, W_(B), W_(A) and W_(C), respectively.

FIG. 17 is a table 505 that illustrates the operation of the equalizingoutput driver 470 of FIG. 16 in response to exemplary values of weights,W_(A), W_(B) and W_(C). In a first example, the pre- and post-tapweights, W_(A) and W_(C), are zero, and the data drive weight, W_(B), isa maximum value (127×I_(REF) in this example). In this configuration,the pre- and post-tap data values do not affect the output signalgenerated by the equalizing output driver 470 and, instead, the datavalue, B, alone determines the output signal. To achieve the ×127 datadrive strength (i.e., I_(REF)×127), the MSBs of the weight value, W_(B),are all high to allocate all seven ×16 sub-drivers 495 within theallocated driver circuit 471 to the data driver pool (illustrated intable 505 by the selection of the data value, B, by each of theallocation control bit pairs, AC₀-AC₆, within the allocated drivercircuit), and all the LSBs of the weight value, W_(B), are high toenable the full ×15 drive strength of the dedicated data sub-driver 477.Thus, a data drive strength of (7×16)+15=127×I_(REF) is achieved. Noneof the sub-drivers within the allocated driver circuit are allocated tothe pre- or post-tap pools, and all the LSBs of the pre- and post-tapweight values are low, thereby disabling signal contributions from thededicated pre- and post-tap sub-drivers 475 and 479.

The second row of table 505 presents a second example of the operationof the equalizing output driver 470 in which, W_(A)=12, W_(B)=102 andW_(C)=13. Because neither of the pre- or post-tap weights is greaterthan 15, none of the sub-drivers 495 within the allocated driver circuit471 are allocated to the pre- and post-tap driver pools. Instead, thededicated pre- and post-tap drivers are enabled to draw ×12 and ×13currents by the setting of the pre- and post tap weight LSBs (i.e.,W_(A)[3:0]=12 and W_(C)[3:0]=13). Because the specified data drivestrength is less than 112 (i.e., the total data drive strength of allthe unallocated sub-drivers 495 within the allocated driver circuit471), one of the sub-drivers 495 within the allocated driver circuit 471is disabled (indicated in FIG. 17 by the selection of ‘0’ by theallocation control bit pair, AC₀), and six sub-drivers 495 are allocatedto the data driver pool, thereby providing a ×96 data drive strength.The dedicated data sub-driver 477 is used to provide the remaining ×6drive strength (i.e., W_(B)[3:0]=6).

Row three of table 505 presents a third example in which W_(A)=23,W_(B)=94 and W_(C)=10. Because the pre-tap weight, W_(A), is greaterthan 15, the dedicated pre-tap sub-driver 475 is insufficient by itselfto provide the specified drive strength. Accordingly, a ×16 sub-driver495 within the allocated driver circuit 471 is allocated to the pre-tapdriver pool (indicated in FIG. 17 by the selection of pre-tap datasource ‘A’, by allocation control bit pair AC₀) to provide a ×16 pre-tapdrive strength, with the remaining ×7 pre-tap drive strength beingsupplied by the dedicated pre-tap sub-driver 475. Because the post-tapweight, W_(C), is less than 16, the specified post-tap drive strength isprovided entirely by the dedicated post-tap sub-driver 479. Finally,because the specified data drive strength is less than 6×16, but greaterthan 5×16, five sub-driver circuits within the allocated driver circuitare allocated to the data driver pool to provide a ×80 data drivestrength, and a value of W_(B)[3:0]=14 is applied to the dedicated datasub-driver 477 to provide the remaining ×14 data drive strength.

Row four of the table ## illustrates another example of the operation ofthe equalizing output driver 470 of FIG. 16, in this case with W_(A)=17,W_(B)=89 and W_(C)=21. In this example, one sub-driver 495 within theallocated driver circuit 471 is allocated to the pre-tap driver pool,another sub-driver 495 is allocated to the post-tap driver pool and fivesub-drivers 495 are allocated to the data driver pool, thereby providingpre-tap, post-tap and data drive strengths of ×16, ×16 and ×80,respectively. The remaining ×1 pre-tap drive strength is supplied by thededicated pre-tap sub-driver 475; the remaining ×5 post-tap drivestrength is supplied by the dedicated post-tap sub-driver 479 and theremaining ×9 data drive strength is supplied by the dedicated datasub-driver 477.

FIG. 18 illustrates an embodiment of an allocation logic circuit 515that may be used to implement allocation logic 493 of FIG. 16. Theallocation logic circuit 515 includes coding circuits 517 ₁, 517 ₂ and517 ₃, shift circuit 519 and control signal generator 521. The codingcircuits 517 receive the MSBs of the pre-tap, data, and post-tap weightvalues, respectively (i.e., W_(A), W_(B) and W_(C)), and, in response,generate decoded pre-tap, data and post-tap values D_(A), D_(B) andD_(C). In one embodiment, illustrated by table 540 of FIG. 19, eachdecoded value includes 2^(N)−1 bits in which the number of high bitscorresponds to the numeric value represented by selected MSBs of thecorresponding weight value (N being the number of weight MSBs).Specifically, in the exemplary decoding shown by table 540, there arethree input bits (i.e. weight bits W[6:4]) and seven (2³−1) constituentbits of the decoded value, D[6:0]. When the numeric value of W[6:4] iszero (i.e., W[6:4]=000b, ‘b’ indicating binary notation), none of thedecoded bits, D[6:0] is high. When the numeric value of W[6:4] is one(i.e., W[6:4]=001b), one of the decoded bits is high (bit D[0] in thisexample). Similarly when the numeric value of W[6:4] is two, two of thedecoded bits are high; when the numeric value of W[6:4] is three, threeof the decoded bits are high; and so forth until the numeric value ofW[6:4] is seven (i.e., W[6:4]=111b) in which case all seven of thedecoded bits, D[6:0] are high. The coding scheme shown in FIG. 19 isreferred to herein as a thermometer code and the coding circuits of FIG.18 are referred to as thermometer coding circuits. Other coding schemesmay be used in alternative embodiments.

The decoded post-tap value, D_(C), is input to the shift circuit 519,along with the MSBs of the pre-tap value (i.e., W_(A)[6:4] in thisexample). In one embodiment, the shift circuit 519 shifts the bitpattern of the decoded post-tap value according to the numeric valuerepresented by the MSBs of the pre-tap value. Thus, as shown in table550 of FIG. 20, when the numeric value of W_(A)[6:4] is zero, thedecoded post-tap value, D_(C)[6:0], is shifted left by zero bitpositions to generate the shifted post-tap value, S_(C)[6:0]. When thenumeric value of W_(A)[6:4] is one, the decoded post-tap value isshifted left by one bit; when the numeric value of W_(A)[6:4] is two,the decoded post-tap value is shifted left by two bits and so forth.Referring to FIGS. 19 and 20, it can be seen that the shifting of thedecoded post-tap value according to the numeric value of the pre-tapMSBs effectively aligns the decoded pre and post-tap values so that highbits within the two values do not fall within the same bit positions.That is, if the shifted post-tap value, S_(C), is logically ORed withthe decoded pre-tap value, D_(A), the number of high bits in theresultant value will be equal to the combined number of high bits withinthe D_(A) and D_(C) values.

Referring again to FIG. 18, the shifted post-tap value, S_(C), is inputto the control signal generator 521 along with the decoded pre-tapvalue, D_(A), and the decoded data value, D_(B). The control signalgenerator 521 includes a number of select logic circuits 523 ₀-523 ₆each of which generates a respective one of the allocation control bitpairs, AC₀[1:0]-AC₆[1:0]. Each select logic 523 circuit receives arespective bit of the decoded pre-tap value, D_(A), the shifted post-tapvalue, S_(C), and the data value D_(B). In one embodiment, theconnections of the constituent bits of the decoded data value, D_(B), tothe select logic circuits 523 is in reverse order relative to the bitconnections of the decoded pre-tap value, D_(A), and shifted post-tapvalue, S_(C). Specifically, select logic circuit 523 receives bit zeroof the decoded pre-tap and shifted post-tap values (i.e., bits D_(A)[0]and S_(C)[0]), but bit six of the decoded data value (i.e., D_(B)[6]).Similarly, select logic circuit 523 ₁ receives D_(A)[1] and S_(C)[1],but D_(B)[5]. Generally stated, if there are N bits within each of thedecoded and shifted values, an i^(th) one of the select logic circuitsreceives bits D_(A)[i], S_(C)[i] and D_(B)[(N−1)−i]. By thisarrangement, any high bits within the decoded data value are effectivelyshifted to the leftmost positions within the overall bit field.Consequently, so long as the total number of decoded bits within values,D_(A), D_(B), and D_(C) is equal to or less than the number ofsub-driver circuits, none of the high bits within the left-shifteddecoded data value will occupy bit positions occupied by high bitswithin the decoded pre-tap value, D_(A) or the shifted post-tap value,S_(C). Note that, in alternative embodiments the same effect may beachieved by shifting the decoded data value or decoded pre-tap valueinstead of the post-tap value and that, similarly, the select logicconnections of the decode pre- or post-tap values may be reversedinstead of the decoded data value connections. In any case, the overallgroup of shifted, decoded values forms a control value, referred toherein as an allocation control word, that indicates the sub-driver pool(pre-tap, post-tap or data) to which sub-drivers within the allocateddriver circuit 471 are to be allocated.

Table 560 of FIG. 21 illustrates, by way of example, the logicaloperation of an i^(th) one of the select logic circuits 523 ₀-523 ₆ ofFIG. 18. Because of the bit shifting achieved by the shift circuit 519and the reversed bit connections of the decoded data value, D_(B), atmost one of the input values, S_(C)[i], D_(B)[6−i] and D_(A)[i] will behigh for a given value of i. If none of the input values is high (as inthe first row of table 560), the two constituent bits of allocationcontrol bit pair, AC_(i) (i.e., AC_(i)[1] and AC_(i)[0]), are both low,thereby selecting the disabled condition for the correspondingsub-driver. If the decoded pre-tap bit, D_(A)[i] is high, ACi[1:0]=01 toallocate the corresponding sub-driver to the pre-tap sub-driver pool(i.e., enable the sub-driver to be controlled by the pre-tap datavalue). If the decoded data bit, D_(B)[6−i] is high, ACi[1:0]=10 toallocate the corresponding sub-driver to the data sub-driver pool, andif the shifted post-tap bit, S_(C)[i] is high, ACi[1:0]=11 to allocatethe corresponding sub-driver to the post-tap sub-driver pool.

FIG. 22 illustrates an exemplary embodiment of a select logic circuit570 that operates in accordance with the logic table 560 of FIG. 21.Logic OR gate 571 receives a shifted post-tap bit S_(C)[i] and a decodedpre-tap bit D_(A)[i] so that AC_(i)[0] is high if either the decodedpre-tap bit or the shifted post-tap bit is high. Logic OR gate 573receives the shifted post-tap bit S_(C)[i] and a decoded data bit,D_(B)[6−i] so that, AC_(i)[1] is high if either the decoded data bit orthe shifted post-tap bit is high.

It should be noted that while the equalizing driver 470 of FIG. 16 hasbeen described as enabling a specific number of sub-driver circuits toone of three driver pools, the equalizing driver may readily be adaptedto enable allocation of any number of sub-driver circuits to any numberof driver pools. In general, if there are N weight values, W₁-W_(N),each corresponding to a different driver pool, P₁-P_(N), to whichsub-driver circuits may be allocated, then each of the weight values maybe decoded to generate decoded values, D₁-D_(P), of which values,D₂-D_(P), may be shifted to generate a set of shifted values, S₂-S_(P),such that none of the high bits within any of the shifted values or thedecoded value D₁ occupy the same bit positions as in another of thevalues. The shifting operation may performed by any type of shiftingcircuit capable of performing the following general operations:

S₂ = D₂  shifted  according  to  D₁S₃ = D₃  shifted  according  to  D₁ + D₂S₄ = D₄  shifted  according  to  D₁ + D₂ + D₃ ⋯S_(P) = D_(P)  shifted  according  to  D₁ + D₂ + … + D_(P − 1)Note that the last shift may be effected by reversing the D_(P) bitconnections to the select logic circuits 523 as in the case of thedecoded data bit connections in FIG. 18. Also, the shift logic may besimplified by limiting the number of shifts of any pre-tap value,post-tap value or data value according to the maximum anticipated numberof sub-drivers needed for the value. For example, one such embodimentincludes one pre-tap sub-driver pool, three post-tap sub-driver pools,and one data driver pool, with a maximum of two sub-driver circuitsbeing allocated to either of the pre- and post-tap sub-driver pools.Finally, the present invention is not limited to shift-based logic forallocation of sub-drivers among different sub-driver pools. In general,any combinatorial logic circuit, state-based logic circuit (e.g., statemachine or processor) or other circuit for allocating sub-drivers todifferent driver pools may be used without departing from the spirit orscope of the present invention. Also, rather than allocating sub-drivercircuits according to decoded weight values, decoded values themselvesmay be provided (e.g., from a configuration circuit or off-chip source)to control the allocation of sub-drivers. For example, values thatdirectly represent the state of the allocation control signals, AC, maybe stored in a configuration circuit or otherwise provided to theequalizing driver of FIG. 16 to control the allocation of sub-driversamong different driver pools.

Although equalizing output drivers have described in reference to FIGS.4-22 in terms of equalizing a data transmission to counteract the affectof ISI from signals transmitted on the same signal path, such equalizingoutput drivers may additionally (or alternatively) be applied tocompensate for cross-talk (e.g., inductive coupling) from signals onneighboring signal paths. For example, any of the equalizing sub-driversdisclosed herein (including allocated sub-drivers) may be controlled bya data value being transmitted on an adjacent signal path to increase ordecrease the drive strength of the subject data transmission tocounteract cross-talk (or other form of interference) from the adjacentsignal path.

Equalizing Receiver with Reduced Parasitic Capacitance

FIG. 23 illustrates an equalizing receiver 600 according to anembodiment of the present invention. The equalizing receiver 600includes a sampling circuit 601 and equalizing driver 603. The samplingcircuit 601 samples a signal received via pad 201 (i.e., from a bus,point-to-point link, or other signaling path) and outputs receive data(RX DATA) for use by other circuitry (not shown) within an integratedcircuit that contains the equalizing receiver 600. The equalizing driver603 includes an input coupled to receive one or more of the data samplesrecovered by the sampling circuit 601, and an output coupled to the pad201. In one embodiment, the equalizing driver includes a plurality ofpost-tap sub-drivers, each for driving an equalization signal onto theoutput line according to a data value received by the sampling circuit.By this operation, the signal level of line 602 is effectively adjustedto counteract the ISI of previously transmitted signals. The equalizingdriver 603 may be implemented using any of the equalizing output driverembodiments described in reference to FIGS. 4-22, with the pre-tap anddata values being omitted or replaced by post-tap values supplied by thesampling circuit 601. Also, as with all the equalizing output driversdiscussed in reference to FIGS. 4-22, the equalizing driver 603 may beused to perform binary-level signal equalization as well as multi-levelsignal equalization. Also, in alternative embodiments, the equalizingdriver 603 may be used to adjust a threshold reference value (i.e., usedto distinguish between signal levels for signal reception purposes)instead of driving an equalizing signal onto the signaling path.

System Application of Equalizing Transceiver

FIG. 24 illustrates a signaling system 650 in which an equalizing driverand/or equalizing receiver according to embodiments described inreference to FIGS. 4-23 may be used. The system 650 may be used, forexample, within a computing device (e.g., mobile, desktop or largercomputer), networking equipment (e.g., switch, router, etc.), consumerelectronics device (e.g., telephone, camera, personal digital assistant(PDA), etc.), or any other type of device in which signal equalizationis beneficial. More specifically, the system 650 may be a memorysubsystem or any other subsystem within such computing device,networking equipment, consumer electronics device, etc.

The system 650 includes a pair of integrated circuits (ICs) 651 and 653coupled to one another via a receive signal path 652 and a transmitsignal path 654. In the embodiment, shown, the signal paths 652 and 654are unidirectional high-speed serial links for conducting serializedtransmissions from one IC to the other. In alternative embodiments,either or both of the links may be bi-directional (i.e., withappropriate circuitry provided to select which of the ICs is enabled totransmit on the link at a given time), and multiples of such signalpaths may be provided to enable transmission of parallel groups ofsymbols (e.g., each group of symbols forming a data or control word(e.g., command, address, etc.) or portion of a data or control packet).Each transmitted symbol may be a binary symbol (i.e., 0 or 1) or, in thecase of a multi-level signaling system, a symbol having more than twopossible states. Also, the receive signal path 652, transmit signal path654, and/or shared transmit-receive signal path may be a multi-drop busthat is coupled to additional ICs. The ICs 651 and 653 may be peers(e.g., each IC is capable of independently initiating a signaltransmission to the other), or master and slave. Also, the relativestatus of the ICs 651 and 653 may change from time-to-time such that oneIC is a master at a first time, then a slave at another time, and/or apeer at another time.

IC 651 is shown in simplified block diagram form and includes anequalizing output driver 659, equalizing receiver 657 (the equalizingreceiver and equalizing output driver together forming an equalizingtransceiver), and application logic 665. In an alternative system inwhich communications between devices 651 and 653 are unidirectional,either the equalizing receiver 657 or equalizing output driver 659 maybe omitted from device 651 (i.e., depending on the signaling direction).Also, though not shown in FIG. 24, an equivalent equalizing receiverand/or equalizing output driver may be included within the device 653.In any case, the equalizing receiver 657, equalizing output driver 659,or both the equalizing receiver 657 and equalizing output driver 659 maybe implemented using any of the equalizing output driver/receiverembodiments described above in reference to FIGS. 4-23.

A configuration circuit 667 (e.g., register, one-time programmablecircuit, non-volatile memory, etc.) may be provided within theapplication logic 665 or elsewhere in IC 651 to store one or moreequalization select values (e.g., weight values or other values thatindicate the relative signal strengths of pre-tap, post-tap and/or datavalues, including data values to be transmitted on neighboring signalpaths). In the embodiment of FIG. 24, for example, a receiverequalization select value 656 (EQSEL-R) is stored in the configurationcircuit 667 and supplied to the equalizing receiver 657, and an outputdriver equalization select value 658 (EQSEL-D) is stored in theconfiguration circuit 667 and supplied to the equalizing output driver659. A similar configuration circuit may be provided within IC 653 toestablish receiver and/or output driver equalization levels. Theequalization select values may be stored within the configurationcircuit 667, for example, during production time (e.g., in a fusible orotherwise one-time programmable store operation) or during systemrun-time. The equalization select values may be generated within the IC651 (e.g., as a result of calibration activity) or, as shown in FIG. 24,received by the equalizing receiver 657 and supplied to the applicationlogic 665 for storage in the configuration circuit 667. The equalizationselect values may also be input to the IC 651 through another accesspath (e.g., test access port or other communication port).

Although two ICs are depicted in FIG. 24 (i.e., ICs 651 and 653), thecircuits within each of the ICs may alternatively be implemented in asingle IC (e.g., in a system-on-chip or similar application), withsignal paths 652 and 654 being routed via metal layers or other signalconducting structures fabricated within the IC. Further, if distinct ICsare provided as shown in FIG. 24, the ICs may be packaged in separate ICpackages (e.g., plastic or ceramic encapsulation, bare die package,etc.) or in a single IC package (e.g., multi-chip module, paper thinpackage (PTP), etc.).

Although the invention has been described with reference to specificexemplary embodiments thereof, it will be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the invention as set forth in the appendedclaims. The specification and drawings are, accordingly, to be regardedin an illustrative rather than a restrictive sense.

1. A signaling circuit comprising: a plurality of driver circuits eachhaving an output coupled to a first output node; a plurality of selectcircuits each having an output coupled to a control input of acorresponding one of the driver circuits, each of the select circuitshaving a plurality of data inputs to receive a plurality of data signalsand a control input to receive a respective one of a plurality of selectsignals, each of the select circuits being adapted to select, accordingto the one of the select signals, one of the plurality of data signalsto be output to the control input of the corresponding one of the drivercircuits; and a dedicated driver circuit that includes a plurality ofoutput drivers coupled to the first output node, each of the outputdrivers including a control input coupled to receive a respective one ofthe plurality of data signals.
 2. The signaling circuit of claim 1,wherein the output drivers of the dedicated driver circuit include biasinputs to receive drive strength select values, each of output driversbeing adapted to generate an output signal in accordance with therespective one of the plurality of data signals and a respective one ofthe drive strength select values.
 3. The signaling circuit of claim 1further comprising an allocation logic to output the plurality of selectsignals.
 4. The signaling circuit of claim 3 wherein the allocationlogic is to decode a plurality of weight values to output the pluralityof select signals.
 5. The signaling circuit of claim 1 wherein at leastone of the plurality of data signals corresponds to a data value to betransmitted by another signaling circuit.
 6. The signaling circuit ofclaim 1 further comprising a sampling circuit coupled to the firstoutput node to receive the plurality of data signals therefrom, thesampling circuit having an output coupled to provide the plurality ofdata signals to the plurality of the data inputs of the select circuits.7. The signaling circuit of claim 1 wherein each of the driver circuitsis a pull-down driver circuit.
 8. The signaling circuit of claim 1wherein each of the driver circuits is a push-pull driver circuit. 9.The signaling circuit of claim 1 wherein each of the driver circuits isa multilevel signal driving circuit.
 10. The signaling circuit of claim1 wherein each of the driver circuits is a differential driving circuit.